Adhesives provide a relatively rapid and convenient way for attaching one type of material to another. Over the years, development of adhesive systems has addressed ways to modify the strength of the adhesive bond between selected materials. Depending on the application, the adhesive of choice may include a pressure sensitive adhesive, a structural adhesive, a hot-melt adhesive or a repositionable adhesive. Regardless of the wide range of available adhesives, the surfaces of some commonly occurring polymer substrates still lack suitable adhesive affinity to form strong bonds with other materials, whether similar or dissimilar. Lack of bonding to surfaces with low adhesive affinity also affects the bonding and retention of surface coatings or printing inks that may be used for surface marking or decoration.
One solution to poor bond formation and poor retention of coatings and inks lies in the use of special treatments to change the condition of a surface by increasing its surface energy. Surface treatments for increased surface energy include oxidizing pretreatments or the use of adhesion promoters. Oxidizing pre-treatments include the use of flame, ultraviolet radiation, corona discharge, and chemical oxidizing agents and the like. Adhesion promoters include chemical compositions containing e.g. solutions of styrene-ethylene/butylene-styrene block copolymers or acrylate polymers in an organic solvent. These compositions improve the bonding of pressure sensitive adhesives and structural adhesives to polymer substrates.
Unfortunately, these methods for raising surface energy are not without problem. For example, chemical oxidizing agents are typically hazardous materials requiring special handling. Surface treatment using flame, corona discharge, etc. usually requires special equipment of a design more suited to sheet or film or web processing than to surface treatment of shaped articles.
Both oxidizing and adhesion promoter pre-treatments provide improvement in bond formation associated with low surface energy polymers such as polyethylene (PE), polypropylene (PP), ethylene-propylene-diene terpolymer (EPDM) and ethylene-alpha olefin copolymers. Substrates of this type may also include ethylene-vinyl acetate copolymers as representative of polymers that are substantially free from ethylenic unsaturation.
Polymeric substrates that have some degree of ethylenic unsaturation also benefit from the use of surface pre-treatments. It is known, for example, that rubbers comprising styrene/butadiene copolymers (SBR) require treatment with chlorinating agents including solutions of halogen donors such as trichloroisocyanuric acid, and N,N-dichlorobenzenesulfonamide, for improved bonding with adhesive bonding agents. Such halogen donors may be incorporated into primers or adhesives separately. The effectiveness of treatment by chlorination appears somewhat limited to substrates having a high level of ethylenic unsaturation such as polybutadiene, polyisoprene, natural rubber, styrene-butadiene copolymers, polychloroprene and the like. Also the addition of halogen donors reduces the stability of some adhesives, including polyurethane adhesives.
Further improvement in bonding to numerous types of substrates may be possible by abrading the surface of a substrate even before applying any pre-treatment. Mechanical means for surface roughening then represents a preliminary step to surface pre-treatments. Regardless of their effectiveness, pre-treatments represent an additional step in the process of manufacturing a product. Additional process steps incur additional time and expense and thereby add undesirable cost to a final product.
The development of primer pre-treatments, primarily in solvent, has yielded some improvement in adhesive bonding to low energy surfaces. U.S. Pat. No. 6,008,286, for example, provides compositions comprising mixtures of hydrocarbon polymers, halogen-substituted hydrocarbon polymers and substituted aliphatic isocyanates which, coated from solvent, improve the bond between low energy substrates and adhesives, coatings, printing inks and the like. The benefits of this primer and related solvent based primer compositions may be diminished by the need to limit volatile organic compounds (VOC) to a level that avoids health hazards and prevents environmental contamination.
Solvent based or aqueous based thermosettable primers may be used, for bond improvement, without a flame or corona preliminary treatment. U.S. Pat. No. 6,001,469 describes primers and topcoats of this type used with e.g. thermoplastic polyolefins (TPO) and reaction injection molded polyurethane (RIM). These materials may be suitably cured on the substrate at temperatures in the region of 130.degree. C. for 30 minutes. Similarly WIPO publication WO 94/28077 describes aqueous-based compositions requiring heat treatment at 130.degree. C. for 40 minutes. It is known (see e.g. R. Ryntz in "Waterborne, High Solids Powder Coatings Symposium," Univ. of Southern Mississippi 1995), that high temperature treatment may also affect the surface morphology of thermoplastic polyolefin polymers. Such changes may be beneficial in some cases, but in others the relatively high temperature for curing may be sufficiently close to the material melting point to produce substrate dimensional changes and associated problems.
Attempts to include primer compositions and/or adhesion promoter compositions with adhesive compositions, have met with limited success. According to an article in the Journal of Coating Technology, 65, No. 827 p. 21 (1993), it is known that chlorinated polyolefin primer compositions do not provide the same level of priming when included as an adhesive additive. Similarly, water based adhesive compositions and primers for polyolefins, described in U.S. Pat. No. 5,298,552, appear primarily useful for applications requiring bonding of polyolefin sheets to porous fibrous substrates such as water-absorbent paper board and similar paper pulp or wood fiber products. Also, in this case, the composition of the adhesive component of the formulations appears to have more impact on the bonding force developed between a polyolefin and absorbent substrate than the chlorinated polyolefin. It appears that common solvents, such as toluene, xylene etc. are unsuitable for use with compositions described in U.S. Pat. No. 5,298,552. Instead, the preferred solvent is relatively uncommon dodecyl benzene.
Known water based primer compositions derive their stability from the use of conventional surfactant materials to keep active components dispersed in the aqueous phase. The amount of surfactant requires careful control. Insufficient surfactant leads to instability and precipitation of the dispersed phase. Addition of too much surfactant produces a primer having poor adhesion to substrates that require priming. Good adhesion is essential to effective performance of priming compositions.
In view of the above described deficiencies associated with the use of known adhesive bonding agents, particularly the use of multiple treatments at substrate surfaces, the present invention has been developed to alleviate these drawbacks and provide further improvement and cost reduction. These enhancements and benefits are described in greater detail hereinbelow with respect to several alternative embodiments of the present invention.